(1) Field of the Invention
The invention relates to a microelectronic mechanical switch (MEMS) device and method of fabrication, and more particularly, to an inductor-capacitor resonant radio 54 frequency (LCR RF) switch and a method of fabricating a LCR RF switch.
(2) Description of the Prior Art
Generally, RF switches, consisting of solid state devices, such as diodes and field-effect transistors (FET), are used in communication systems applications. For very high frequencies of about 1 GHz, these diode and FET devices are typically fabricated using expensive GaAs technology. However, RF switches fabricated using diodes and FET devices demonstrate high insertion loss and low isolation when the working frequency exceeds 1 GHz. In addition, the value of the isolation decreases with frequency.
Recently, microelectronic mechanical (MEMS) technology has been used for the fabrication of RF switches. A MEMS switch features significant advantages in its small size as measured in the operating wavelength. MEMS has potentially lower costs since IC batch processing can be used. As an example, a microelectronic mechanical switch (MEMS) may be constructed which uses electrostatic force to flex a thin membrane and thereby cause the switch to be opened or closed. Such devices are fabricated with dimensions in the range of 100""s of microns and can be integrated onto an integrated circuit device. Since an electrostatic force is used, the switch can be controlled using only a voltage and very little, or no, current. Therefore, it consumes virtually no power. This is an important advantage for portable communication systems, such as hand-held mobile phones or other wireless communication devices, where power consumption is recognized as a significant operating limitation.
Referring now to FIG. 1, an MEMS device is illustrated in cross section. There is shown, in highly simplified form, a MEMS switch over which the present invention is an improvement. It is to be understood in this regard that no portion of FIG. 1 is admitted to be prior art as to the present invention. Rather, this highly simplified diagram is provided in an effort to provide an improved understanding of the problems which are overcome by the invention.
In this example, the device is fabricated on a substrate 10. An insulating layer 12 overlies the substrate 10 to isolate the switch from the substrate 10. A metal microstrip 14 overlies the dielectric layer 12. The metal microstrip 14 may be designed to carry a microwave signal, for example. A dielectric layer 18 overlies the metal microstrip 14. A bridge structure is formed by the combination of the bridge posts 22 and the membrane 26. The bridge posts 22 are formed straddling the metal microstrip 14. The membrane 26 is fixed to the bridge posts 22 at each end. The bridge posts 22 and membrane 26 may comprise metallic materials. The membrane 26 is very thin such that an electrostatic force can cause it to flex. The distance between the membrane 26 and the dielectric layer 18 is an air gap.
This MEMS device has two states of operation. In the UP state, the membrane 26 is suspended above the dielectric layer 18 as shown. In this state, there is very little capacitive coupling between the bridge structure and the metal microstrip 14. At microwave frequencies, the small capacitor between the bridge structure 22 and 26 and the metal microstrip 14 forms a large impedance value. Therefore, very little of the microwave energy is transferred into the bridge structure 22 and 26.
Referring now to FIG. 2, the DOWN state of operation of the MEMS device is shown. If a sufficiently large, DC bias voltage exists between the membrane 26 and the metal microstrip 14, the electrostatic force will cause the thin membrane 26 to flex toward the microstrip 14. At maximum deflection, the membrane 26 contacts the dielectric layer 18 as shown. In this state, the capacitive coupling between the microstrip 14 and the bridge structure 22 and 26 is much higher than in the non-flexed state. The large capacitance forms a much smaller impedance value for the microwave signal. Therefore, much of the microwave energy is conducted into the bridge structure 22 and 26.
As can be seen, the MEMS device functions as a variable capacitor on the microstrip 14 node of the circuit. When the membrane is in the UP state, the switch is OFF. The signal flowing on the microstrip 14 continues to flow along the microstrip 14. When the membrane is down, due to the DC bias, the switch is ON. The signal is redirected through the capacitor and into the bridge membrane 26 and posts 22.
The figure of merit for the MEMS device is the ratio of the insertion loss in the DOWN state and the isolation during the UP state. The MEMS exhibits very low insertion loss and very high isolation. The resonant frequency of the MEMS device determines the particular frequency at which the high isolation can be achieved. The resonant frequency depends upon the capacitance in the DOWN state and the small inductance of the bridge structure. Note that the area of the capacitor formed between the membrane 26 and the microstrip 14 in the DOWN state is proportional to the area of the bridge contacting the dielectric layer 18, which is, in turn, proportional-to the contact length L1.
Referring now to FIG. 3, an equivalent circuit model for the MEMS device is shown. In this model, the MEMS device is configured as a shunt switch. The bridge posts are connected to ground. The microstrip is modeled as the lumped impedance elements Z0 48. The MEMS bridge is modeled as a variable capacitor Cb 52, a series inductance Lb 56 and a series resistance Rs 60. The variable capacitor Cb 52 represents the aforementioned variable capacitive coupling due the deflection of the membrane. The series inductance Lb 56 and series resistance Rs 60 are due to the physical characteristics of the membrane and bridge posts. When the MEMS switch is in the UP state, Cb 52 is small, and most of the microwave energy is conducted past the switch. When the MEMS switch is in the DOWN state, Cb 52 is large, and most of the microwave energy is conducted through the switch to ground.
Note that, in the DOWN state, the series capacitance Cb 52 and the series inductance Lb 56 result in a series resonant frequency given by:
xcfx89=1/(LbCb)xc2xd.
Typically, the MEMS device can be optimized for useful operating frequencies of greater than about 5 GHz. However, for frequency bands below 5 GHz, this MEMS device exhibits too low of an isolation. This is because the bridge inductance Lb is usually very small and is not adjustable.
Finally, the fabrication technique for this MEMS capacitor RF switch is difficult to control. One fabrication technique is to spin on a photoresist layer prior to the deposition of the thin membrane layer. The photoresist layer is then removed to form the deflection gap. Unfortunately, it is very difficult to uniformly control the thickness of spun on photoresist. The yield of qualified MEMS devices in a wafer will therefore be limited.
Several prior art approaches disclose MEMS devices and methods to form MEMS devices. Z. J. Yao et al, xe2x80x9cMicromachined Low-Loss Microwave Switches,xe2x80x9d IEEE Journal of Microelectromechanical Systems, Vol. 8, No. 2, June 1999, pp. 129-134, discloses an MEMS device for microwave applications. A capacitively-coupled switch is formed where a dielectric layer separates a bottom electrode from a suspended membrane. J. B. Muldavin et al, xe2x80x9cHigh-Isolation Inductively-Tuned X-Band MEMS Shunt Switches,xe2x80x9d 2000 IEEE MTT-S International Symposium Digest, June 2000, pp. 169-172, discloses an inductively-tuned MEMS device. Straight transmission lines are used to add inductance to the shunt-configured, MEMS switch circuit between the bridge and ground. U.S. Pat. No. 5,619,061 to Goldsmith et al teaches various configurations of micromechanical microwave switches. Dielectric, metallic, and combination membranes are disclosed. Both direct coupling and capacitive coupling are taught. U.S. Pat. No. 6,069,540 to Berenz et al discloses a MEMS device with a pivot pin structure. A rigid beam is used to improve reliability. U.S. Pat. No. 5,880,921 to Tham et al teaches a switched capacitor bank formed using MEMS technology. U.S. Pat. No. 6,074,890 to Yao et al discloses a MEMS device where the motion of a signal device is coupled to the motion of a slave device. The preferred fabrication uses a backside dry etch to release the suspended MEMS devices and mechanical couplers. U.S. Pat. No. 6,020,564 to Wang et al teaches a MEMS device where the actuation voltage is reduced by leveraging a small actuating beam movement into a large longitudinal beam movement. U.S. Pat. No. 5,578,976 to Yao teaches a MEMS device with a cantilevered beam-fabricated from dielectric material.
A principal object of the present invention is to provide an effective and very manufacturable MEMS-based inductor-capacitor resonant RF switch and method of fabrication.
A further object of the present invention is to provide a MEMS-based inductor-capacitor resonant RF switch by combining a MEMS variable capacitor and a spiral inductor.
A yet further object of the present invention is to combine a MEMS variable capacitor and a spiral inductor to thereby create an inductor-capacitor resonant RF switch with improved operating characteristics below 5 GHz.
Another further object of the present invention is to improve the operating characteristics of a MEMS variable capacitor by adding an upper electrode to increase capacitive coupling between the membrane and the microstrip.
Another further object of the present invention is to apply the inductor-capacitor resonant RF switch in a shunting configuration.
Another further object of the present invention is to apply the inductor-capacitor resonant RF switch in a series configuration.
Another further object of the present invention is to apply the inductor-capacitor resonant RF switch in a multiple-channel, series configuration.
Another further object of the present invention is to provide a method of fabricating a microelectronic mechanical switch device with improved uniformity.
Another yet further object of the present invention is to provide a method of fabricating a microelectronic mechanical switch device using a dual damascene process.
In accordance with the objects of this invention, a new inductor-capacitor resonance RF switching device is achieved. The device comprises a microelectronic mechanical switch and a spiral inductor. The microelectronic mechanical switch comprises, first, a first dielectric layer-overlying a substrate. A down electrode overlies the first dielectric layer. A second dielectric layer overlies the down electrode. An up electrode may overlie the down electrode with the second dielectric layer therebetween. A bridge post overlies the first dielectric layer and does not contact the down electrode or the up electrode. Multiple bridge posts may be used. Finally, a membrane is suspended over said down electrode. At least one end of the membrane is fixed to the top of a bridge post. An electrostatic potential between the membrane and the down electrode will cause the membrane to flex down toward the down electrode. This flexing of the membrane will cause the capacitance of the switching device to vary. The spiral inductor comprises a metal line configured in a spiraling pattern with a first end connected to a bridge post and a second end forming an output node.
Also in accordance with the objects of this invention, a new multiple channel, series configured LCR-RF switching circuit is achieved. The circuit comprises a plurality of MEMS capacitor and spiral inductor pairs. In each pair, a first end of each spiral inductor is connected to a bridge post of each MEMS capacitor. The down electrodes of all the MEMS capacitors are connected to a single input signal. A second end of each spiral inductor forms a plurality of output signals. Activation of any MEMS capacitor in any pair causes the input signal to flow to the output signal for the pair.
Also in accordance with the objects of this invention, a new method to fabricate a microelectronic mechanical switch device is achieved. A down electrode is provided overlying a substrate with a first dielectric layer therebetween. A second dielectric layer is provided overlying the down electrode layer. A first silicon dioxide layer is deposited overlying the second dielectric layer. A silicon nitride layer is deposited overlying the first silicon dioxide layer. A second silicon dioxide layer is deposited overlying the silicon nitride layer. The second silicon dioxide layer, the silicon nitride layer, the first silicon dioxide layer, and the second dielectric layer are patterned to form deep trenches for planned bridge posts. The second silicon dioxide layer and the silicon nitride layer are patterned to form shallow trenches for the planned membrane. The shallow trenches connect to the deep trenches. A metal layer is deposited overlying the second silicon dioxide layer, the silicon nitride layer, the first silicon dioxide layer, and the second dielectric layer to fill the deep trenches and the shallow trenches. The metal layer is polished down to the silicon nitride layer to complete the bridge posts and the membrane. The second silicon dioxide layer, the silicon nitride layer and the first silicon dioxide layer are etched away to release the membrane and to complete the microelectronic mechanical switch device in the manufacture of the integrated circuit device.